tree(fix): Validate source indices correctly on moves (#21552)

## Description

This PR fixes the validation of moves when there is a specified source
array. Previously, the validation was using the destination array for
validation unconditionally instead of the source when it was passed.

Author: taylorsw04

packages/dds/tree/src/simple-tree/arrayNode.ts

@@ -773,17 +773,20 @@ abstract class CustomArrayNodeBase<const T extends ImplicitAllowedTypes>
 		fieldEditor.remove(removeStart, removeEnd - removeStart);
 	}
 	public moveToStart(sourceIndex: number, source?: TreeArrayNode): void {
-		const field = getSequenceField(this);
+		const sourceArray = source ?? this;
+		const field = getSequenceField(sourceArray);
 		validateIndex(sourceIndex, field, "moveToStart");
 		this.moveRangeToIndex(0, sourceIndex, sourceIndex + 1, source);
 	}
 	public moveToEnd(sourceIndex: number, source?: TreeArrayNode): void {
-		const field = getSequenceField(this);
+		const sourceArray = source ?? this;
+		const field = getSequenceField(sourceArray);
 		validateIndex(sourceIndex, field, "moveToEnd");
 		this.moveRangeToIndex(this.length, sourceIndex, sourceIndex + 1, source);
 	}
 	public moveToIndex(index: number, sourceIndex: number, source?: TreeArrayNode): void {
-		const field = getSequenceField(this);
+		const sourceArray = source ?? this;
+		const field = getSequenceField(sourceArray);
 		validateIndex(index, field, "moveToIndex", true);
 		validateIndex(sourceIndex, field, "moveToIndex");
 		this.moveRangeToIndex(index, sourceIndex, sourceIndex + 1, source);

packages/dds/tree/src/test/simple-tree/arrayNode.spec.ts

@@ -110,6 +110,17 @@ describe("ArrayNode", () => {
 		});
 
 		describe("moveToStart", () => {
+			it("move element to start of empty array", () => {
+				const schema = schemaFactory.object("parent", {
+					array1: schemaFactory.array(schemaFactory.number),
+					array2: schemaFactory.array(schemaFactory.number),
+				});
+				const { array1, array2 } = hydrate(schema, { array1: [], array2: [1, 2, 3] });
+				array1.moveToStart(1, array2);
+				assert.deepEqual([...array1], [2]);
+				assert.deepEqual([...array2], [1, 3]);
+			});
+
 			it("move within field", () => {
 				const array = hydrate(schemaType, [1, 2, 3]);
 				array.moveToStart(1);
@@ -144,6 +155,17 @@ describe("ArrayNode", () => {
 		});
 
 		describe("moveToEnd", () => {
+			it("move element to end of empty array", () => {
+				const schema = schemaFactory.object("parent", {
+					array1: schemaFactory.array(schemaFactory.number),
+					array2: schemaFactory.array(schemaFactory.number),
+				});
+				const { array1, array2 } = hydrate(schema, { array1: [], array2: [1, 2, 3] });
+				array1.moveToEnd(1, array2);
+				assert.deepEqual([...array1], [2]);
+				assert.deepEqual([...array2], [1, 3]);
+			});
+
 			it("move within field", () => {
 				const array = hydrate(schemaType, [1, 2, 3]);
 				array.moveToEnd(1);
@@ -178,6 +200,17 @@ describe("ArrayNode", () => {
 		});
 
 		describe("moveToIndex", () => {
+			it("move element to start of empty array", () => {
+				const schema = schemaFactory.object("parent", {
+					array1: schemaFactory.array(schemaFactory.number),
+					array2: schemaFactory.array(schemaFactory.number),
+				});
+				const { array1, array2 } = hydrate(schema, { array1: [], array2: [1, 2, 3] });
+				array1.moveToIndex(0, 1, array2);
+				assert.deepEqual([...array1], [2]);
+				assert.deepEqual([...array2], [1, 3]);
+			});
+
 			it("move within field", () => {
 				const array = hydrate(schemaType, [1, 2, 3]);
 				array.moveToIndex(0, 1);